As industry has turned to an extensive utilization of complex processing and control equipment, a concomitant need has arisen for specialized treatment of the environment within which this equipment operates. Exemplary of such equipment are computer installations which exhibit relatively high heat emission rates representing a generally continuous (year around) load to air conditioning equipment, the operational demands upon which have heretofore been typically seasonal in nature. In addition to requiring accurate removal of generated heat loads, such installations further require control over the relative humidity within their zonal environment.
Heretofore, the air conditioning industry has responded to requirements for providing accurate temperature and humidity control within the enclosed regions of computer rooms and the like, on the one hand, with the somewhat brute force approach of operating typical refrigeration components to continuously treat air within the zone surrounding the equipment. As might be expected, the energy demands associated with such an approach do not represent an insignificant cost factor. In another approach, advantage has been taken of seasonally lower outdoor (ambient) temperatures by a controlled mixing of cooler outdoor air with recirculated air. This commingling of air permits an intermitant deactivation of energy consuming refrigeration components.
Humidity control typically is carried out through the use of humidifiers to add moisture generally at the output region of air conditioning apparatus, while dehumidification, on the other hand, is a more elaborate process. Typically, dehumidification requires the operation of refrigeration components to remove moisture and this operation usually is associated with a reheating of the thus dehumidified but cooled air to regain the required zone target or "set point" temperature. One disadvantage attendant with the use of outside air to contribute to cooling stems from the general tendency of industry to locate computer rooms and the like well within the internal regions of buildings. As a consequence, access to outside air can be achieved only through the use of extensive and bulksome ducting, a requirement rendering the otherwise achieved energy conservation impractical and economically unsound. However, should the location disadvantage be overcome, typically encountered variations in ambient air humidity have been found to counteract the energy gains from utilizing outdoor air at lower temperature, inasmuch as a greater consumption of energy is required to carry out humidification and dehumidification than initially conserved in using the outside air. This outside air also will contain undesirable contaminants such as dust and the like.
The first practical system introduced to industry wherein effective energy conservation was achieved taking advantage of lower outside or ambient air temperatures is described in U.S. Pat. No. 3,525,385. This system provides an outdoor heat exchanger which operates in conjunction with a liquid heat exchanging medium, such as glycol or the like, which is pumped through conduits into the building retaining the enclosed air conditioned region and at which point this fluid is utilized both to provide heat exchange within the condenser units of a refrigeration system and, alternately, to provide cooling through the use of a liquid cooling coil interposed within the air flow of the internally disposed air conditioning equipment. Thus, as outdoor temperatures drop below predetermined levels, full advantage is taken of those lower levels to accommodate the heat loads imposed from computer equipment and the like. Alternately, in warmer seasons, the higher outdoor temperatures can be utilized to provide a reheat function utilizing the heat exchange liquid in conjunction with now operating refrigeration components. As industry has seen fit to adapt this energy conserving system, the term "free-cooling" has been found to be used to describe such an energy saving feature.
Even greater efficiencies or reductions in energy consumption have been recognized in an improved air conditioning system based upon the above, first practical free cooling approach. Described, for example, in application for U.S. Patent Ser. No. 779,743, by Ralph C. Liebert, filed Mar. 21, 1977, and entitled: "Liquid Refrigeration System for an Enclosure Temperature Controlled Outdoor Cooling or Preconditioning". This improved system significantly expands the capability of air conditioning apparatus to utilize the effects of outdoor temperatures which vary above or below the desired predetermined or set point temperature level of the air within the zone to be conditioned. For example, the system utilizes substantially all of the cooling capability of the outdoor air by precooling recirculated or return air within the system to the extent possible before such air is treated by energy consuming refrigeration components operating in conjunction with the evaporator coils and the like. The improved system further takes advantage of heat generating compressors to carry out a reheating function in conjunction with the operation of refrigeration components for dehumidification.
By taking advantage of a broadened range of outdoor temperatures, this improved air conditioning system requires control features which are capable of responding to a correspondingly broad range of temperature conditions both within the zone wherein air is conditioned as well as at the outdoor heat exchanger locale. Thus, an accurate control responsive to react to small changes in heat loadings as well as to ambient temperature variations is desired to gain highest efficiencies for the system. Additionally, such controls should operate under a logic wherein energy conservation is enhanced, i.e. wherein components of highest power consumption are utilized most sparingly.
A variety of control circuits have been introduced in the past for the purpose of controlling earlier air conditioning systems. For example, reference is made to U.S. Pat. Nos., 3,522,451; 3,714,980; 3,883,757 and 3,844,475.